1. Field of the Invention
The present invention relates to a mobile communications system with hand-off control, and in particular, relates to a CDMA mobile communications system with hand-off control.
2. Description of the Related Art
In a mobile communications system, usually, a communications area is divided into many wireless communications areas, each called a “cell”, and base stations are provided in each area. Then, a mobile station is accommodated in one of the base stations in the communications area, and is connected to another communications terminal through the base station.
FIG. 1 shows the configuration of a general mobile communications system. In this specification, a CDMA mobile communications system, which has recently been spread, is used as an example.
In each cell, a base station device (BTS: base station transceiver subsystem) is provided. The base station device can simultaneously accommodate a plurality of mobile stations (MS), and transmits/receives radio data to/from a mobile station. Each base station device is accommodated in a base station controller (BSC).
The base station controller accommodates a plurality of base station devices and controls them. The base station controller is also connected to a mobile services switching center (MSC) and transmits/receives data to/from it, as requested. The mobile services switching center is connected to another switching center and a public switched telephone network (PSTN) is composed of the plurality of switching centers.
In a CDMA mobile communications system, a different spread code is assigned to each mobile station. Then, data that are encoded using the spread code and transmitted between a mobile station and a base station device.
A mobile station often moves from one cell to another cell while communicating with another communications terminal. A transition from a state where a mobile station is accommodated in a base station device into a state where the mobile station is accommodated in another base station device, when the mobile station moves from a cell to another cell, is called “hand-off” (or “hand-over”).
A hand-off operation is largely classified into two categories: a “soft hand-off” and a “hard hand-off”. In the soft hand-off, even when moving from one cell to another cell, a mobile station is always connected to at least one base station device. Therefore, in the soft hand-off, a channel between the mobile station and base station device is never disconnected, and, accordingly, communications are never interrupted. In the system shown in FIG. 1, the soft hand-off occurs, if a mobile station moves between cells that are controlled by the same base station controller and if the same frequency can be allocated.
In the hard hand-off, when moving from one cell to another cell, the mobile station is temporarily disconnected from the base station device and then is connected to another base station device. In this case, synchronization must be established again between the mobile station and the base station device. Thus, in the hard hand-off, a channel between a mobile station and a base station device is temporarily disconnected and, accordingly, communications are interrupted. Therefore, a user is often disconcerted. In the system shown in FIG. 1, the hard hand-off occurs when a mobile station moves between cells each of which is controlled by a different base station controller. In other words, the hard hand-off occurs when a mobile station crosses an “accommodation boundary”. The hard hand-off also occurs when a mobile station moves between cells, to each of which a different frequency is allocated.
FIG. 2 shows the configuration of the existing mobile communications system, and the configuration covers the vicinity of a boundary between areas each of which is controlled by a different base station controller. In this example, a base station controller 1 (BSC#1) accommodates base station devices 11 (BTS#A) and 12 (BTS#B), and a base station controller 2 (BSC#2) accommodates a base station device 13 (BTS#C). Here, it is assumed that the base station devices 11 through 13 control the cell-a through cell-c, respectively, and the same radio frequency is allocated to all of the cells.
If a mobile station moves from cell-a to cell-b, or from cell-b to cell-a, a soft hand-off occurs since both the base station devices 11 (BTS#A) and 12 (BTS#B) are accommodated in the base station controller 1 (BSC#1). However, if the mobile station moves from cell-b to cell-a or from cell-c to cell-b, a hard hand-off occurs since each of the base station devices 12 (BTS#B) and 13 (BTS#C) is accommodated in a different base station controller.
Therefore, if as shown in FIG. 3, the mobile station moves back and forth between cell-b and cell-c several times while communicating, many hard hand-offs occur, and the user will be disconcerted and feel unassured.
As described above, in the existing mobile communications system, a hard hand-off is easy to occur in the vicinity of a boundary between particular communications areas, and a user feels uncomfortable.